1. Technical Field
The present invention relates to a thermopile infrared sensing element and a temperature measuring device using the same.
2. Description of the Related Art
A known temperature measuring device uses an infrared sensing element which detects infrared radiation emitted from a heat source and converts it into an electric signal. Examples of known infrared sensing elements include a thermopile type, a pyroelectric type, a borometer, and the like. The thermopile infrared sensing element utilizes the Seebeck effect of a thermopile comprising a thermocouple or a plurality of thermocouples connected in series, for detecting a temperature change due to infrared radiation or absorption as thermo electromotive force. The pyroelectric infrared sensing element detects a temperature by using a change in floating charge due to polarization according to the infrared thermal energy, i.e. a pyroelectric effect, of a base material comprising ceramic or the like. The borometer comprises a thermosensitive resistor comprising a thin film or a thin wire of a metal or the like so that a thermal change in resistance value of the resistor is detected.
Of these sensing elements, the pyroelectric infrared sensing element requires a shutter provided on an optical path, for alternately measuring the temperatures of a surface to be measured and a reference point because polarization occurs only at the moment of application of thermal energy. Therefore, it is difficult to miniaturize the pyroelectric infrared sensing element and improve measurement accuracy. The pyroelectric infrared sensing element can thus be used for applications such as a xe2x80x9cbody sensing elementxe2x80x9d of an automatic door and the like, which are little required to have a small size or high measurement accuracy. However, the pyroelectric infrared sensing element is unsuitable for a temperature measuring device such as a clinical thermometer or the like, which is required to measure temperatures with high accuracy and have a small size and low cost.
Furthermore, the pyroelectric and thermopile infrared sensing elements require detecting a relative temperature difference from a reference temperature, while the borometer has the advantage that an absolute temperature can be measured. However, the barometer has many causes of error, such as self heating due to measuring current, current noise, etc., thereby causing difficulties in improving the measurement accuracy. The borometer also requires a bias current that complicates the structure, and thereby complicates handling.
On the other hand, the thermopile infrared sensing element does not use such a transient phenomenon as utilized by the pyroelectric type and such excess measuring current as flowing through the barometer, and thus the thermopile infrared sensing element can stably detect or measure temperatures. Furthermore, the thermopile infrared sensing element can be miniaturized by using the semiconductor manufacturing process at low cost, and is thus suitable for a temperature measuring device such as a clinical thermometer or the like, which is required to be small in size and inexpensive.
FIG. 21 shows an ear-type clinical thermometer 100 as a temperature measuring device using a thermopile infrared sensor or infrared sensing element. The ear-type clinical thermometer 100 comprises a body case 11, an infrared receiving sensing unit 200 which receives infrared radiation from the ear to output a voltage signal according to the temperature of the ear, i.e. the temperature of the tympanic membrane, and a circuit board 3 on which various electronic parts and circuits are mounted. On the circuit board 3, various electronic parts 4C are mounted to form a temperature deriving circuit 400 for determining the temperature of the tympanic membrane based on the output voltage of the infrared receiving sensing unit 200, or the body temperature based on the temperature of the tympanic membrane. Furthermore, LCD 5 for displaying the determined temperature, and the like, and a power source 6 for supplying electric power to each of the units are also mounted on the circuit board 3.
The infrared receiving sensing unit 200 comprises a case 211 having a cylindrical shape which projects forward (to the right of the drawing) to permit the front end to be inserted into the lughole, a wave guide 206 contained in the case 211, and an infrared sensing chip 210 arranged at the base of the wave guide 206 to face the front end of the case 211. The front end of the case 211 is open, and covered with an infrared transmitting probe cap 25, and the front opening of the wave guide 206 is covered with an infrared transmitting film 23 for preventing entrance of dust particles, etc, the film 23 being supported by a film holding O-ring 24. Therefore, when the front end of the case 211 is inserted into the lughole, infrared radiation entering the infrared receiving sensing unit 200 is guided to the infrared sensing chip 210 by the wave guide 206 so that the infrared sensing chip 210 receives the infrared radiation corresponding to the temperature of the tympanic membrane to output a voltage signal according to the infrared radiation. Therefore, when a temperature measurement switch SW4 of the ear-type clinical thermometer 100 is pushed, the body temperature can be measured by the infrared receiving sensing unit 200 through infrared radiation.
Namely, the clinical thermometer 100 is a wave guide type in which infrared radiation entering from a heat source (the lughole) S is transmitted through the wave guide 206 arranged in front of the infrared sensing chip 210 to be guided to the infrared sensing chip 210, as shown in FIG. 22. The temperature rise due to the infrared radiation is converted into a voltage by the thermopile of the infrared sensing chip 210 and then output.
FIG. 23 shows the infrared sensing chip 210. The infrared sensing chip 210 comprises a thermopile infrared sensor 209, and a thermistor 211, both of which are mounted on a package substrate 212. In the infrared sensing chip 210, the thermistor 211 is used for determining the reference temperature of the thermopile formed in the infrared sensor 209, i.e. the temperature of the cold junction. These components are contained in a package case 213 to unify the components. The package case 213 further comprises an infrared filter 208 composed of silicon or the like and provided in a window through which infrared radiation enters, for cutting-off visible light and transmitting infrared radiation.
FIG. 24 is a perspective view schematically showing the infrared sensor or infrared sensing element 209. The infrared sensor 209 comprises a base 80 comprising a portion (thin film portion) 802 in which only a thin film is left by etching a silicon substrate to form a hollow at the center of the lower surface or the bottom of the silicon substrate, and a thick wall portion 801 in which the silicon substrate remains unetched. Namely, the infrared sensor 209 has a structure in which a hollow portion KW is formed at the center of the base 80 from the lower side to form a membrane at the top of the base 80. Furthermore, gold black is deposited at the top of the thin film portion 802 by sputtering, evaporation, or the like to form an infrared absorber 81 comprising a black body for absorbing infrared radiation. The infrared absorber 81 absorbs infrared radiation to cause a change in temperature.
The infrared sensor 209 further comprises a plurality of thermocouples 82 with high sensitivity, which are provided on every side of the infrared absorber 81. The hot junction 83 of each of the thermocouples 82 is arranged near the infrared absorber 81 or to be overlapped with it, and the cold junction 84 of each thermocouple 82 is arranged in the peripheral thick wall portion 801 in which the silicon substrate remains. Although electromotive force occurs between the hot junction 83 and the cold junction 84 of each thermocouple 82 according to a temperature rise of the infrared absorber 81, the electromotive force produced by a single thermocouple 82 is not sufficient. Therefore, the infrared sensor 209 comprises a plurality of thermocouples 82 which are connected in series to form a thermopile 85 so that a voltage between both ends serving as a terminal (for example, +terminal) 86 and a terminal (for example, xe2x88x92terminal) 87 can be output.
More specifically, as shown in FIG. 25, each of the thermocouples 82 comprises two types of conductors of aluminum (Al) 91 and polysilicon (Sixe2x80x94P) 92. The polysilicon 92 is deposited in a line shape. The end of each of the polysilicon conductors 92, which is arranged near the infrared absorber 81, i.e. which is arranged in the thin film portion 802, is joined to the aluminum wiring 91 to form the hot junction 83. On the other hand, the end of each polysilicon conductor 92, which is arranged in the thick wall portion 801 functioning as a heat sink, is joined to the aluminum wiring 91 to form the cold junction 84. Each aluminum wiring 91 connects the hot junction 83 and the cold junction 84 of the adjacent polysilicon conductors 92, resulting in a state where the thermocouples 82 are connected in series.
FIG. 26 is a block diagram schematically showing the functional configuration of the ear-type clinical thermometer 100. The ear-type clinical thermometer 100 comprises the temperature deriving circuit 400 to which a temperature sensing voltage Vt, across both terminals 86 and 87 of the thermopile 85 of the infrared sensor 209, is input to produce a temperature value T (based on the voltage Vt) applied to the LCD 5. The temperature deriving circuit 400 comprises an amplifier 41 for amplifying the temperature sensing voltage Vt, an A/D converter 42 for converting the output from the amplifier 41 to a digital value, CPU 44 for performing numerical processing of the digital value to determine the temperature corresponding to the temperature sensing voltage Vt, and RAM 43 serving as a working area for each processing in the CPU 44. The temperature deriving circuit 400 detects the electromotive force produced between the hot junctions 83 and the cold junctions 84 of the thermopile 85 of the infrared sensor 209 according to a change in temperature to determine the temperature difference xe2x96xa1T between the hot junctions 83 and the cold junctions 84 based on the output voltage Vt. In order to determine the temperature of the infrared absorber 81 to calculate the body temperature T (herein, the temperature of the tympanic membrane), the temperature (reference temperature) of the cold junctions 84 must be determined for correction.
Therefore, the infrared sensing chip 210 contains the thermistor 211 so as to detect the reference temperature Tr of the infrared sensor 209. The temperature deriving circuit 400 further comprises an amplifier 45 for amplifying the output current It of the thermistor 211, and an A/D converter 46 for converting the output to a digital value to supply a digital signal to the CPU 44. The CPU 44 can thus correct the temperature difference xe2x96xa1T obtained by the signal from the thermopile 85 with the reference temperature Tr obtained by the thermistor 211 to determine the body temperature T.
However, as shown in FIG. 23, the thermistor 211 simply measures the temperature on the side of the infrared sensor 209, i.e. the environmental temperature around the infrared sensor 209 within the infrared detecting chip 210. Therefore, the temperature detected by the thermistor 211 is different from the actual temperature of the cold junctions 84, thereby possibly causing great error in the body temperature T determined as described above.
The thermistor 211 is apparently suitable for measuring the reference temperature in view of the fact that the absolute temperature can be measured because the resistivity changes with temperature. However, the temperature characteristic is nonlinear, and thus an appropriate compensating circuit is required. Also, the range of precise temperature measurement is limited. Therefore, in use of the ear-type clinical thermometer 100, the range of room temperature which permits precise measurement is possibly limited, and thus the body temperature cannot be always easily measured with high accuracy depending upon the environment of measurement.
Accordingly, an object of the present invention is to provide an infrared sensing element and a temperature measuring device using the same which permit an improvement in accuracy of temperature measurement when the temperature is measured by using the above-described thermopile infrared sensing element. Another object of the present invention is to provide an infrared sensing element and a temperature measuring device using the same which permit temperature measurement with ease and high accuracy under any environment.
An infrared sensing element of the present invention comprises a base comprising a thin film portion and a thick wall portion arranged around the thin film portion, a thermopile comprising a plurality of thermocouples connected in series so that the cold junctions thereof are located at the top of the thick wall portion, and the hot junctions thereof are located at the top of the thin film portion, and reference temperature sensing means provided with a thermosensitive portion in contact with the thick wall portion. In the infrared sensing element of the present invention, the thermosensitive portion is arranged in contact with the thick wall portion of the base, which serves as a heat sink of the thermopile. Therefore, the temperature of the thick wall portion in contact with the cold junctions of the thermopile can be detected directly by the thermosensitive portion, and thus the temperature of the cold junctions can be detected substantially accurately even when the environment around the infrared sensing element changes.
Therefore, a temperature measuring device comprising the infrared sensing element of the present invention, and means for determining temperature from the output voltage of the thermopile based on the reference temperature determined by the reference temperature sensing means is capable of measuring temperature with high accuracy. The infrared sensing element of the present invention is a thermopile infrared sensing element, and permits mass-production of small size elements at low cost by utilizing a semiconductor manufacturing process. Therefore, by using the infrared sensing element of the present invention, a compact temperature measuring device can be provided at low cost, which can measure temperature with high accuracy.
Even when the thermosensitive portion comprises a thermistor, the accuracy of the reference temperature is improved to improve the accuracy of temperature measurement. However, it is difficult to incorporate the thermistor in contact with the base. On the other hand, a PN junction, for example, a diode, can be incorporated into a semiconductor substrate, and used as the thermosensitive portion because a forward voltage drop substantially linearly changes depending upon temperature. Therefore, the base of at least the thick wall portion of the base preferably comprises a semiconductor substrate so that the thermosensitive portion comprises the PN junction formed on the semiconductor substrate. As a result, an infrared sensing element comprising a compact thermosensitive portion can be formed. Also, like the thermopile, the thermosensitive portion can be formed by the semiconductor process, thereby decreasing the manufacturing cost.
Furthermore, in the thermistor, the temperature characteristic nonlinearly changes, and thus the range of temperature measurement is limited, thereby causing the restriction that the body temperature cannot be accurately measured by a conventional clinical thermometer using a thermistor unless the room temperature is about 15xc2x0 C. or more. In contrast, by using the PN junction as the thermosensitive portion, the temperature can be easily measured in any place at any time independently of the environment because the temperature characteristic is linear. Therefore, the infrared sensing element of the present invention comprises the PN junction as the thermosensitive portion, and temperature determining means include means for determining the reference temperature based on the forward voltage drop of the PN junction, whereby a compact temperature measuring device with high measurement accuracy within a wide temperature range can be provided at low cost.
By providing a plurality of PN junctions as the thermosensitive portion, the influence of a reverse saturation current on a forward voltage drop can be canceled by determining a difference between the forward voltage drops of these PN junctions. Therefore, the temperature can be more accurately determined, thereby improving the measurement accuracy of the reference temperature, and the accuracy of temperature measurement by the infrared sensing element of the present invention. Therefore, the infrared sensing element of the present invention comprises a plurality of PN junctions as the thermosensitive portion, and temperature determining means include means for determining the reference temperature based on the difference between the forward voltage drops of the plurality of PN junctions, whereby a temperature measuring device with higher measurement accuracy can be provided.
Although the plurality of PN junctions may be arranged in any portion of the semiconductor substrate, the PN junctions are preferably arranged in parallel near the semiconductor substrate in order to prevent differences in characteristic between the PN junctions. The parallel arrangement of the PN junctions enables the use of a common conductor layer as the conductor layers on one side in the plurality of PN junctions, thereby improving the space efficiency and decreasing the number of electrodes. Therefore, the infrared sensing element of the present invention can be further decreased in size. For example, in use of a plurality of diodes, a single anode region, and a plurality of cathode regions extending in parallel to the anode region can be provided.
Furthermore, the PN junction can be formed below each of the cold junctions to bring the thermosensitive portion nearer to the cold junction, whereby the reference temperature can be more correctly detected. It is thus possible to further improve the accuracy of temperature measurement. Where a plurality of PN junctions having a plurality of cold junctions arranged to form the thermopile are formed along the periphery of the thin film portion, the average reference temperature of the cold junctions can be obtained, thereby obtaining the more accurate reference temperature and improving the accuracy of temperature measurement. In forming such a long PN junction, electrodes are preferably respectively formed on the upper surfaces of first and second conductor layers of the PN junction so as to extend along the first and second conductor layers, thereby decreasing a potential difference in the same conductor as much as possible.
Furthermore, a second substrate may be provided on the thick wall portion of the base to be joined to the thick wall portion, and the thermosensitive portion may be provided on the base and/or the second substrate at the junction between the a base and the second substrate. When the second substrate comprises an infrared lens or wave guide, the temperature of the infrared lens or wave guide can be obtained at the same time as detection of the reference temperature. Therefore, temperature correction for lens characteristics or light collecting characteristics can be made to improve the measurement accuracy.
When the second substrate comprises a semiconductor substrate, the PN junctions can be formed on the junction of the second substrate to form the thermosensitive portion. It is thus possible to measure the reference temperature with the thermosensitive portion in direct contact with the thick wall portion of the base, and save the space for arranging the thermosensitive portion on the base side, further decreasing the size of the infrared sensing element. Particularly, the infrared lens or a substrate joined to the infrared lens is suitable for the second substrate. The infrared lens can efficiently guide the infrared radiation emitted from the heat source to the thin film portion where the hot junctions of the thermopile are arranged at the top thereof, and limit the angle of incidence of the infrared radiation received by the thin film portion. Therefore, the sensitivity of temperature measurement can be improved. Also, the influence or interference of infrared radiation due to external heat can be decreased by joining the infrared lens to the thick wall portion of the base. Therefore, in use of the infrared lens, the infrared lens is preferably joined to the base to decrease factors of error, thereby further improving the measurement accuracy. Furthermore, as described above, the junction of the infrared lens is used as the space for the thermosensitive portion, whereby the element can be made compact to decrease the measurement error due to the temperature difference between the reference temperature and the lens temperature.
In order to improve the sensitivity of temperature measurement, the infrared absorber is preferably laminated on the thin film portion to cover at least the hot junctions or the vicinities thereof. With the infrared absorber provided, the infrared absorber absorbs infrared radiation to increase the temperature, thereby increasing the temperature difference between the thin film portion and the thick wall portion, and increasing the output voltage of the thermopile. As a result, the sensitivity of temperature measurement can be further improved. This is thus effective for use of the infrared lens.
Since the infrared lens can be formed by etching a semiconductor substrate, the infrared lens itself may be used as the second substrate, or a junction substrate comprising a semiconductor substrate may be held as the second substrate between the infrared lens and the base. In any case, the PN junction can be provided at the junction to form the thermosensitive portion, providing the compact infrared sensing element enabling an improvement in accuracy of temperature measurement. In forming the PN junction at the junction, a plurality of PN junctions are preferably arranged in parallel as described above. In this case, a common conductor layer is formed for the plurality of PN junctions to make the sensing element further compact. Also, the PN junctions are formed above the cold junctions to permit the measurement of the reference temperature with higher accuracy. When the junction is formed along the periphery of the thin film portion, the PN junctions are formed at the junction or a junction adjacent thereto to improve the measurement precision of the reference temperature. In this case, electrodes are preferably respectively formed in contact with the upper surfaces of the first and second conductor layers of each PN junction so as to extend along the first and second conductor layers at the junction or the base or the semiconductor substrate of the base which is opposed to the junction.
On the other hand, in order to improve the temperature measurement accuracy of the thermopile, it is preferable to decrease the amount of heat conduction between the hot and cold junctions to secure the large temperature difference between both junctions. Therefore, a hole passing through the thin film portion or a recess is preferably provided between the electrodes or conductors, which constitute each thermocouple, to further thin the thin film portion, decreasing the cross section of the thin film portion between the hot and cold junctions. Furthermore, the conductors which constitute each thermocouple, and the conductors for connecting the thermocouples in series are laminated on the thin film portion to increase the area of the thin film portion, which can be cut off by the hole or slit, or which can be thinned, thereby further decreasing the amount of heat conduction. It is effective to also form a hole in each of the conductors for connecting the hot and cold junctions to decrease the amount of heat conduction, or further thin the conductor. Thus, the large temperature difference between both junctions can be maintained, and the electromotive force of each thermocouple can be increased, improving the measurement sensitivity. Furthermore, such a hole or slit can easily be formed by using the semiconductor manufacturing process, without deteriorating the productivity of the infrared sensing element. By using the infrared sensing element of the present invention, a compact temperature measuring device can be provided at low cost, which can easily accurately measure temperatures at any time in any place independently of environment. Therefore, the temperature measuring device of the present invention is suitable as a clinical thermometer, and an ear-type clinical thermometer with high accuracy and ease of handling can be provided at low cost, in which a cylindrical portion is provided at the front end so that it can be inserted into the lughole, and the infrared sensing element is provided so that the thin film portion faces the front end of the cylindrical portion. In the infrared sensing element comprising the infrared absorber laminated thereon, the infrared absorber is preferably arranged to face the front end of the cylindrical portion.